Resin transfer molding (RTM) and resin infusion (RI) are ideally solvent-free processes for making composite parts which can have significant cost advantages over traditional autoclave methods of composite fabrication since an autoclave is not required and there are no volatiles to manage. RTM and RI enable fabrication of highly complex shapes that would otherwise be extremely difficult to fabricate using hand lay-up autoclave techniques. Typically, these processes involve the placement of a woven preform or mat (i.e. glass, carbon, polymer, etc.) in the mold cavity. The molten resin is subsequently injected or infused at an elevated temperature into the mold whereby it permeates through the woven preform. This step is usually performed under vacuum in a sealed mold so there is little opportunity for volatile components to escape. After sufficient time to allow complete wet-out of the preform, the mold is subsequently heated to a higher temperature whereby the resin reacts to crosslink the material. During this step, external pressure is often applied to the mold by means of hydrostatic pressure. It is important that there are no volatiles present either in the form of residual solvent or chemical components of the resin system as they will cause void formation in the composite part. This is a difficult feature to achieve in a resin system for RTM and/or RI processes. Commercial resins such as vinyl esters, epoxies and bismaleimides are available that are processable by RI and/or RTM and provide good mechanical performance; however, these materials are limited in their use temperatures relative to the aromatic imide based materials described herein.
Phenylethynyl containing amines have been used to terminate imide oligomers [F. W. Harris, A. Pamidimuhkala, R. Gupta, S. Das, T. Wu, and G. Mock, Poly. Prep., 24 (2), 325, 1983; F. W. Harris, A. Pamidimuhkala, R. Gupta, S. Das, T. Wu, and G. Mock, J. Macromol. Sci.-Chem., A21 (8 and 9), 1117 (1984); C. W. Paul, R. A. Schultz, and S. P. Fenelli, xe2x80x9cHigh-Temperature Curing Endcaps For Polyimide Oligomersxe2x80x9d in Advances in Polyimide Science and Technology, (Ed. C. Feger, M. M. Khoyasteh, and M. S. Htoo), Technomic Publishing Co., Inc., Lancaster, Pa., 1993, p. 220; U.S. Pat. No. 5,138,028 (Aug. 11, 1992) to National Starch and Chemical Co.; R. G. Byrant, B. J. Jensen, and P. M. Hergenrother, Poly. Prepr., 34 (1), 566, 1993; U.S. Pat. No. 5,412,066 (1995) to National Aeronautics and Space Administration]. Imide oligomers terminated with ethynyl phthalic anhydride [P.M. Hergenrother, Poly. Prep., 21 (1), 81, 1980], substituted ethynyl phthalic acid derivatives [S. Hino, S. Sato, K. Kora, and O. Suzuki, Jpn. Kokai Tokyo Koho JP 63, 196, 564. Aug. 15, 1988; Chem. Abstr., 115573w, 110, (1989)], and phenylethynyl containing phthalic anhydrides [P. M. Hergenrother and J. G. Smith, Jr., Polymer, 35(22)4857 (1994); U.S. Pat. No. 5,567,800 (1996) to National Aeronautics and Space Administration, J. E. McGrath and G. W. Meyer, U.S. Pat. No. 5,493,002 (1996) to Virginia Tech Intellectual Properties, Inc., J. A. Johnson, F. M. Li, F. W. Harris and T. Takekoshi, Polymer, 35(22)4865 (1994), T. Takekoshi and J. M. Terry, Polymer, 35(22)4874 (1994), R. J. Cano and B. J. Jensen, J. Adhesion, 60, 113 (1997)] have been reported. Imide oligomers containing pendent substituted ethynyl groups [F. W. Harris, S. M. Padaki, and S. Varaprath, Poly, Prepr., 21 (1), 3, 1980 (abstract only), B. J. Jensen, P. M. Hergenrother, and G. Nwokogu, Polymer, 34 (3), 630, 1993; B. J. Jensen and P. M. Hergenrother, U.S. Pat. No. 5,344,982 (Sep. 6, 1994); J. W. Connell, J. G. Smith, Jr. R. J. Cano and P. M. Hergenrother, Sci. Adv. Mat. Proc. Eng. Ser., 41, 1102 (1996); U.S. Pat. No. 5,606,014 (Feb. 25, 1997) to National Aeronautics and Space Administration] and pendent and terminal phenylethynyl groups, [J. G. Smith, Jr., J. W. Connell and P. M. Hergenrother, Polymer, 38(18), 4657 (1997)] have been reported.
A high temperature resin system designated as phthalonitrile has been developed that exhibits low melt viscosity. This material however suffers from poor melt stability and lacks suitable toughness after thermal cure. Simple laminates have been fabricated by RTM, however mechanical properties were low and the resin exhibited microcracking upon thermal cycling (D. E. Duch, Sci. Adv. Mat. Proc. Eng. Ser., 44, 705 (1999).)
The present invention constitutes a new composition of and method for preparing a mixture of imide random co-oligomers and imide compounds that exhibit a unique combination of properties that make them particularly useful in the fabrication of composite parts via RTM and/or RI processes. These materials can be readily synthesized and isolated in a solvent-free and moisture-free form, exhibit the proper flow, melt stability and lack of volatile formation to allow for processing using RTM and/or RI techniques. Upon thermal curing, these materials exhibit sufficient thermal stability, toughness and mechanical properties so as to be useful as composite matrix resins in high performance applications. They are also useful as adhesives, coatings, films, foams and moldings (both filled and unfilled).
According to the present invention, a composition of and method for making high performance resins that are processable by resin transfer molding (RTM) and resin infusion (RI) techniques were developed. Materials with a combination of properties, making them particularly useful for the fabrication of composite parts via RTM and/or RI processes, were prepared, characterized and fabricated into moldings and carbon fiber reinforced composites. These materials are particularly useful for the fabrication of structural composite components for aerospace applications. This method produces aromatic imide based resins that are processable into complex composite parts using RTM and/or RI.
The method for making high performance imide resins for RTM and/or RI processes is a multi-faceted approach. It involves preparation of a mixture of products from a combination of aromatic diamines with aromatic dianhydrides at relatively high stoichiometric offsets and endcapping with latent reactive groups. The combination of aromatic diamines includes at least over approximately 50 molar percent of a flexible diamine. It also may include less than approximately 50 molar percent of a rigid diamine. Alternatively, or in conjunction with the rigid diamine, a diamine containing pendent phenylethynyl groups comprising less than approximately 20 molar percent of the total diamine combination can also be utilized. This combination of monomers provides a balance of properties that impart flexibility (melt flow) with those that impart rigidity for sufficiently high glass transition temperature (Tg) and results in a mixture of products, in the imide form, that exhibit a stable melt viscosity of less than approximately 60 poise below approximately 300xc2x0 C. The use of a high stoichiometric offset produces a mixture of products consisting of different molecular weight imide oligomers and simple imide compounds. This mixture of products can be detected through gel permeation chromatographic (GPC) analyses that indicate multi-modal molecular weight distributions. Additional experimental observations also indicate that there are components in the mixture of products that significantly contribute to the low melt viscosities exhibited by the materials described herein.
The selection of the monomers used to prepare the imide oligomers desires a balance of monomers that impart flexibility (i.e. melt flow) with those that impart rigidity (high cured Tg). The resultant mixture of products from their reaction should exhibit a stable melt viscosity of less than approximately 60 poise below approximately 300xc2x0 C. In addition, the mixture of products formed does not contain components that are volatile under the processing conditions. A flexible diamine is described herein as an aromatic diamine that imparts a low melt viscosity to the uncured resin and without its use the uncured resins would have melt viscosities too high for use in resin transfer molding. A flexible diamine may contain three or more phenyl rings whereby the phenyl rings are connected by divalent radicals such as oxygen, sulfur, 2,2xe2x80x2-isopropylidene and 2,2xe2x80x2-hexafluoroisopropylidene and wherein at least one of the divalent radicals is located in the ortho or meta position on a phenyl ring. Some flexible diamines include, but are not limited to, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-minophenoxy)benzene and 1,3-bis(3-aminothiophenoxy)benzene. A rigid diamine is described herein as an aromatic diamine that may contain one or more phenyl rings and that when used, in combination with a flexible aromatic diamine, in the preparation of resin transfer molding materials described in the present invention, provides cured resins with high Tgs (more than approximately 230xc2x0 C.). When the rigid diamine contains a single phenyl group, the amino groups can be located in any substitution pattern (1,2-, 1,3- or 1,4-) on the phenyl ring. When the aromatic diamine contains two phenyl rings and the phenyl rings are connected by a covalent bond or any divalent radical, the amino groups can be located in any substitution pattern on the phenyl rings. When the aromatic diamine contains more than two phenyl rings and the phenyl rings are connected by a covalent bond or a divalent radical, the amino groups can be located in any substitution pattern different from that of the flexible aromatic diamines described above. Other aromatic rings such as naphthalene, 1,3,4-oxadiazole or quinoxaline can be substituted for the inner phenyl ring. The materials described herein preferably contain more diamine than dianhydride, therefore, the diamine components are the biggest contributing factor to melt flow behavior. High stoichiometric offsets can be defined as an offset resulting in low molecular weight (approximately 1500 theoretical molecular weight). The latent reactive endgroup may be 4-phenyethynylphthalic anhydride. Alternatively, monoamines containing phenylethynyl groups such as, 3-phenylethynylaniline or 4-amino-4xe2x80x2-phenylethynylbenzophenone could be used as endcapping agents.
An extension of the invention described herein would be to employ alternate synthetic routes commonly used to produce amide acids and imides to prepare the materials described herein. Another extension would be to prepare amide acid and imide oligomers and random co-oligomers of comparable molecular weight containing only pendent phenylethynyl groups. Alternative synthethic routes to prepare amide acids and imides containing phenylethynyl groups have been performed. For example, the reaction of dianhydride(s) and diamine(s) and 4-phenylethynylphthalic anhydride in m-cresol containing isoquinoline at elevated temperature gives the imide directly. Phenylethynyl containing amide acid oligomers and random co-oligomers can also be prepared by the reaction of diamine(s) with an excess of dianhydride(s) and endcapped with a monofunctional amine containing phenylethynyl groups under a nitrogen atmosphere at room temperature in N-methyl-2-pyrrolidinone (NMP). Phenylethynyl terminated imide oligomers and random co-oligomers can be prepared by the reaction of the half alkyl ester of aromatic tetracarboxylic acids with aromatic diamines and endcapped with the half alkyl ester of a phenylethynyl substituted phthalic acid or a phenylethynyl amine by heating in NMP. Phenylethynyl containing imide oligomers and random co-oligomers prepared by the alkyl ester route can also be prepared by heating neat or in solvents such as m-cresol. Phenylethynyl terminated imide oligomers and random co-oligomers can be prepared by the polymerization of monomeric reactants approach by heating a mixture of a diamine(s), the half alkyl ester of a dianhydride(s) and endcapped with the half alkyl ester derivative of 4-phenylethynylphthalic anhydride.
The method described herein produces materials that are useful as adhesives, coatings, films, foams, moldings, powders and composite matrices. In powder form, the imides are useful for making adhesive tape and carbon/graphite fiber prepreg (dry tow) that is subsequently converted into dry prepreg (tape or ribbon) and used in an automated tape placement process without the use of solvents. The powders and/or pellets are particularly useful in making composite parts by RTM and/or RI processes. To demonstrate this technology, high quality, complex composites parts such as F-frames, skin stringer panels, I-beams, sine wave spars and window frames have been fabricated by RTM and RI using the materials described herein.